Allelopathy can play a beneficial role in various cropping systems such as mixed cropping, multiple cropping, cover cropping, crop rotations, and minimum and no–tillage systems. In a 5–yr field study with sunflower (Helianthus annuus L.)–oat rotation, the weed density increase was significantly less in sunflower plots than in control plots. It was found that sunflower plants possess chemicals, which inhibit the growth of common weed species. Macias et al., (1999) reported some sesquiterpene lactones with germacranolide and guaianolide skeletons and heliannuol from different cultivars of sunflower. These authors discussed their potential role as natural herbicides. Fujii et al. (1992) reported that velvetbean (Mucuna prursens (L.) DC. var. utilis) is a good candidate to smoother weeds. They identified L-DOPA (L-3,4-dihydroxyphenylalanine) as the chemical responsible for velvetbean phytotoxicity.